Extrusion dies have been found useful for the production of ceramic honeycomb structures for use as catalytic converters in the exhaust stream of internal combustion engines. As the use of ceramic honeycomb structures has broadened to catalytic converters in a wider range of engine types and in stationary emission control, to chemical process structures, to refining structures, and to alternative catalytic systems and uses, the need for ceramic honeycomb structures with greater cell density per unit of transverse cross-sectional area and/or greater geometric surface area per cell has strained the usefulness of the current extrusion die technology. The beneficial action of catalytic converters is a function of the surface area available to the exhaust stream for interaction with the noxious output of internal combustion engines. Significant advances in ceramic honeycomb structure technology may thereby be achieved by increasing the cell density (per square inch or centimeter) of the structure or by increasing the surface area per cell.
Formation of the extrusion die defines the subsequent geometry and plurality of longitudinally extending cells possible for ceramic honeycomb structure. Therefore, the limits of extrusion die technology necessarily limits the ceramic honeycomb structure technology.
The prior art contains various types of extrusion dies. U.S. Pat. No. 3,790,654 discloses a machined extrusion die formed from a unitary die block. A plurality of machined or cut, stacked plates, with their longer dimensions extending in the longitudinal flow direction through the die, are disclosed in U.S. Pat. Nos. 3,790,654 and 4,465,652 as a laminated blade extrusion die to be used to produce the honeycomb structure. U.S. Pat. No. 4,550,005 discloses a compound die (i.e. a type having two or more bonded pieces whose longer dimensions extend transverse of the longitudinal flow direction through the die) of the compound feed type with a replaceable perforated plate attached on the inlet portions of feed passageways. Common to all of these extrusion dies are feed holes which communicate with all of the intersecting or crisscrossing grid-like discharge slots.
U.S. Pat. Nos. 4,298,564 and 4,354,820 show compound dies of the compound slot type with intersecting wider slots connecting between the feed holes and the intersecting discharge slots. Each wider slot is fed by feed holes. Each narrower discharge slot is longitudinally fed directly only by one of the wider slots.
Japanese Unexamined Patent Application Publication 52-8761 shows a feed of the inner ends of the discharge slots only through separated lateral connecting passages extending from the inner ends of the feed holes and ending at the inner ends of the slots.
All of these prior art die designs (with the one limited exception) fundamentally rely on feed holes longitudinally aligned and in communication with all of the discharge slots. The prior art teaches the need for a feed hole which is functionally related to discharge slots by feeding material to each discharge slot to insure proper batch flow to form fully knitted honeycomb structures. The relationship of the large number of holes to total lateral slot expanse in FIG. 8 of U.S. Pat. No. 4,259,057 clearly suggests, in light of the other functional teaching, that only a limited deviation from the fundamental prior art teaching is permissible in that regard. Thus, as the number of slots in a die is increased in order to increase the cell density of products produced with a die, the prior art teaching requires a commensurate increase in feed holes aligned therewith to maintain proper flow.